This invention relates to a DC arc welding apparatus. More particularly, the invention relates to a DC arc welding apparatus provided with a control circuit capable of controlling the rise of the welding current.
One DC arc welding apparatus of this type is shown in FIG. 1, wherein reference numeral 1 designates a DC power source including a welding transformer, 2 is a switching element for controlling the output current of the power source 1, 3 and 4 are the output terminals of the welding apparatus, 5 is a contact chip and 6 is a consumable welding electrode (hereinafter referred to as "the wire") to which electric power is supplied through the contact chip 5. The material 8 to be welded (hereinafter referred to as "the base metal") is connected to the output terminal 4, and arcs 7 are established between the wire 6 and the base metal 8. A current detecting element 9 such as a shunt resistor is coupled to an output current control circuit 17 for controlling the output signal of the detecting element 9 in accordance with the output of an arithmetic circuit 11. The arithmetic circuit 11 is constituted by an operational amplifier 12, a resistor 13 coupled between an inverted terminal of the amplifier 12 and an output voltage setting circuit 10 such as a variable resistor, a resistor 14 coupled between the output terminal 3 and the inverted input terminal of the amplifier 12, a resistor 15 coupled between an output terminal of the amplifier 12 and the inverted input terminal thereof and a capacitor 16 coupled in parallel with the resistor 15. Reference numeral 18 denotes a drive circuit for amplifying the output of the control circuit 17 and controlling the on-off operation of the switching element 12.
The operation of this DC arc welding apparatus shown in FIG. 1 will now be described.
An output voltage set signal e.sub.s of the output voltage setting device 10 and an output voltage feedback signal e.sub.vf are subjected to comparison and amplification in the amplifier 12. The output signal e.sub.vo of the arithmetic circuit 12 and the output current signal e.sub.if of the output current detecting element 9 are processed in the output current control circuit 17 to produce a signal e.sub.io which is applied to the following drive circuit 18 so that the signal e.sub.if corresponding to the output signal e.sub.vo is always obtainable. The drive circuit 18 operates to drive the switching element 2 in response to the above described signal e.sub.io. When the switching element 2 is repeatedly rendered conductive (ON) and non-conductive (OFF), current flows, in order, from the DC power circuit 1 through the switching element 2, the output terminal 3, the contact chip 5, the wire 6, the arc 7, the base metal 8, the output terminal 4 and the output current detecting element 9 back to the DC power circuit. The above output current is fed-back to the output current control circuit 17 by the output current detecting element 9 as an output current signal e.sub.if. In this circuit, the output current set signal is the output signal e.sub.vo of the arithmetic circuit 11. The output current set signal e.sub.vo is determined so that the output voltage feedback signal e.sub.vf is made constant irrespective of the arc condition (the output voltage set signal e.sub.s being constant). Therefore, the voltage between the output terminals 3 and 4 is constant regardless of the load therebetween. However, upon the occurrence of a short-circuit between the wire 6 and the base metal 8, if the control circuit operates to supply a large current in the wire 6 momentarily to thereby make the voltage therebetween constant, a current larger than that required flows in the wire 6. This causes the wire to melt partially and spread in all directions (hereinafter referred to as "spattering"). Due to such spattering, the distance between the wire 6 and the base metal 8 immediately after the short-circuit is removed (hereinafter referred to as the arc length) becomes long resulting in an unstable arc condition including the extinction of the arc. Further, the spattered metal may adhere to the base metal 8, and it is difficult to remove the same. Consequently, a time delay element such as a capacitor 16 is provided in the arithmetic circuit 11 so that the output signal e.sub.vo varies with the delay time in response to an abrupt variation of the output voltage feedback signal e.sub.vf. As a result, the output current is not increased abruptly but rather gradually in response to the formation of the short-circuit between the wire 6 and the base metal 8. Shown in FIG. 2 are the relations among the output voltage feedback signal e.sub.vf, the output voltage setting signal e.sub.s, the output signal e.sub.vo and the output current I. The above described apparatus is more fully described in co-pending application Ser. No. 401,774, filed July 26, 1982, and examples of the output current control circuit 17 and the drive circuit 18 are also described in detail therein.
It is considered that if the variation rate of the output current in the case of a short-circuit between the wire 6 and the base metal 8 is determined so that the rate in a small current range is larger than that in a large current range, a stable arc condition is obtainable over the entire operating range and spatters hardly occur. However, with the apparatus described above, since the variation rate is made constant, if the capacitor 16 is provided to make the arc stable in the small current region, a large quantity of spatters may occur in the large current region. On the other hand, if the capacitor 16 is provided to make the arc stable in the large current region, the arc condition may become unstable in the small current region.